This invention relates to a composition for absorbing electromagnetic radiation and, particularly, for absorbing the electromagnetic radiation over a wide range of frequencies from sub-GHz to GHz (106 to 1010 hertz) band regions.
It is well-known that magnetic materials such as soft ferrites, carbonyl iron or metallic iron may be used for absorbing electromagnetic radiation (hereinafter referred to as xe2x80x9cEMRxe2x80x9d). In order to use as an EMR absorbing barriers, such materials are divided into fine particulates or powders, dispersed in a suitable binder and then fabricated into sheets. Since such magnetic materials are capable of absorbing EMR mainly in frequency ranges in the order of several GHz, absorption of EMR in sub-GHz frequency ranges requires the finished sheets to have so large thickness that makes the sheets too heavy and difficult to manufacture.
JP-A-10308596 discloses an EMR absorbing laminate capable of operating in both VHF and UHF band regions. The laminate comprises, in the direction of incident radiation, a dielectric layer containing barium titanate, a second layer containing carbonyl iron or ferrites and a short circuiting metal plate. The first dielectric layer may be formed of sintered or resin-bound barium titanate particles having a thickness from 0.1 to 1.5 mm. The second layer may be formed of resin-bound carbonyl iron particles having a thickness from 1.0 to 4.0 mm. Alternatively, the second layer may be formed of sintered or resin-bound ferrite particles having a thickness from 4.0 to 10.0 mm. This laminate suffers from the same disadvantage as above in terms of large thickness and weight and difficulties in manufacture.
JP-A-11289188 assigned to the assignee of this application discloses an EMR absorbing composition comprising titanium slug. This composition exhibits maximum EMR attenuation at a frequency of around 4.3 GHz.
A need exists, therefore, for an EMR absorbing composition which may be fabricated into an EMR absorbing sheet or layer capable of absorbing EMR over a wide range of frequencies from sub-GHz to several GHz band regions with a relatively small thickness in non-composite structure.